In an automobile engine, to purify the exhaust gas, the air-fuel ratio is usually controlled to a stoichiometric air-fuel ratio, and a three-way catalytic converter that simultaneously oxidizes HC, CO and reduces NOx is often installed in the exhaust pipe.
In such a catalytic converter, a platinum type catalyst having platinum as its principal component is used. At the high temperature of the exhaust gas, platinum oxidizes when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and the catalyst performance temporarily decreases as a result. This temporary deterioration progresses under high temperature, lean conditions, but the catalyst is restored when, at high temperature, the air-fuel ratio reverts to a stoichiometric or rich condition.
in this context, Jikkai Sho 63-128221 published in 1988 by the Japanese Patent Office for example discloses a mechanism that restores a platinum type catalyst by making the air-fuel ratio rich when catalyst oxidation reaches a certain level.
However, recovery of the platinum catalyst requires not only a rich air-fuel ratio, but also a high temperature, and if the temperature falls during the recovery process, recovery of the catalyst is delayed. In this mechanism, catalyst recovery also takes place under running conditions when exhaust temperature is low such as when the engine is running on low load. This causes catalyst recovery to be delayed, and if the engine continues to run at a rich air-fuel ratio at low temperature for a long time, it increases fuel consumption and worsens the exhaust gas composition.